1. Field of the Invention
The present invention relates to a free-fall detector device for and to a free-fall detection method.
2. Description of the Related Art
As is known, the use of microelectromechanical or MEMS (Micro-Electro-Mechanical-Systems) inertial sensors is becoming increasingly common for different reasons in a wide range of portable electronic devices, such as cell phones, laptops and palmtops, camcorders, digital cameras, and the like. In these devices, in particular, the inertial sensors are often advantageously exploited for making free-fall detectors, which enable prevention or at least reduction of the damage deriving from impact by bringing sensitive parts into a safety configuration. Owing to the increasingly marked miniaturization, for example, many of the devices listed above are equipped with hard disks, which call for an extremely high mechanical precision and have very delicate parts. The surfaces of the disks and the read/write heads, in use, are practically in contact and can be seriously damaged as a result of impact caused by a fall. A free-fall detector in many cases enables the read/write heads to be moved away from the surfaces of the disks in due time and then to be parked in a safety area before the electronic device completes its fall. Upon impact, then, the likelihood of serious damage occurring is considerably reduced.
Clearly, the effectiveness of a free-fall detector depends upon the precision and the speed in detecting the falling condition.
In this connection, it should be recalled that inertial sensors of the types used in free-fall detectors can detect not only accelerations and decelerations of the device in which they are incorporated, but also the intensity of the action of the gravitational field with respect to one or more detection axes. When the device is in rest conditions, said intensity will evidently be all the greater, the closer the direction of the detection axis approaches the vertical (i.e., parallel to the acceleration of gravity). Instead, the intensity of the action of the gravitational field is substantially zero if the detection axis is horizontal. However, when a device incorporating an inertial sensor is in a free-fall state, the acceleration detected is substantially zero irrespective of the direction of the detection axis.
Consequently, known free-fall detectors normally use microelectromechanical inertial sensors with three independent axes so as to be sensitive to accelerations in whatever way oriented, and the criterion used for detecting free fall is that the value of the detected acceleration is lower than a threshold value irrespective of the direction.
In greater detail, in a first known type of free-fall detector, three acceleration signals corresponding to the acceleration components according to the three detection axes are continuously converted into numeric signals and supplied to the processing unit. Here the numeric signals are squared and summed together. Then, the processing unit extracts the square root of the sum, which represents the total acceleration value to which the free-fall detector is subjected (and hence also a device that incorporates said free-fall detector), and compares the total acceleration with a threshold close to zero. If the absolute value of the total acceleration is lower than the threshold value, a free-fall condition is signaled. Free-fall detectors of this type are very precise because the exact value of the magnitude of the total acceleration is determined, but are slow in so far as numerous calculations are required (squaring, summation, extraction of square root, comparison). Furthermore, the computation resources are markedly exploited because the acceleration must be continuously monitored. Consequently, conflicts or slowing-down of operations may occur if the processing unit is shared with the device that incorporates the free-fall detector. To prevent conflicts and slowing-down, it is necessary to provide an independent processing unit, which, however, entails a considerable waste of space.
In a second known type of free-fall detectors, which is also based upon inertial microelectromechanical sensors with three independent detection axes, the signals acceleration according to each of the detection axes are converted into numeric signals and compared directly and separately with a threshold close to zero. The free-fall condition is detected when the numeric signals for the three detection axes are all lower than the threshold.
In this case, the calculation speed is favored (only three operations of comparison with a threshold are necessary), at the expense, however, of precision. In fact, the absolute value of the total acceleration is not calculated, and the separate comparisons are reliable only when the total acceleration is substantially parallel to one of the detection axes. Otherwise, a total acceleration with absolute value clearly greater than the threshold value (indication contrary to the free-fall condition) may have components according to the detection axes separately lower than the threshold itself. The free-fall detector could hence respond by signaling a false free-fall condition (false positive). To prevent an excessive number of false positives very low thresholds can be used, but by so doing the sensitivity decreases.
In any case, the threshold is anisotropic in space.